Optical measurement apparatus and method for optical measurement

ABSTRACT

A liquid scintillation counter  10  serving as an optical measurement apparatus includes: an HPD  24,  a charge amplifier  26,  a voltage amplifier  28,  a comparator  30,  a counter  32,  a multi-channel analyzer  34,  a display  38,  and the like. The HPD  24  has a photocathode  24   a  and an APD  24   b  for outputting a signal that corresponds to the number of incident photons. The comparator  30  outputs a logic pulse signal, serving as a comparison result signal, only when the signal outputted from the HPD  24  and amplified by the charge amplifier  26  and voltage amplifier  28  is larger than a prescribed threshold value. This threshold value is set larger than an output signal that is outputted when a single photoelectron is emitted from the photocathode  24   a  and smaller than another output signal that is outputted when two or more photoelectrons are emitted.

TECHNICAL FIELD

The present invention relates to an optical measurement apparatus suchas a scintillation counter, particle counter, and the like, and a methodfor optical measurement such as a method of scintillation counting, amethod of particle counting, and the like.

BACKGROUND ART

For example, an optical measurement apparatus employing aphotomultiplier tube is well known in the art for measuring weak light,such as fluorescent light emitted from a scintillator. Thephotomultiplier tube includes a photocathode that emits photoelectronsthat correspond to the amount of light of an incident beam, and amultiplying unit for amplifying and outputting the photoelectronsemitted from the photocathode. Accordingly, the amount of light of theincident beam can be measured by counting the number of pulse currentsoutputted from the photomultiplier tube.

However, when this optical measurement apparatus is used to performhigh-precision measurements, dark current pulses emitted due to thermalfluctuations and the like, act as noise. One technique for overcomingsuch a problem is a method of simultaneous measurement employing twophotomultiplier tubes, wherein the output pulse is deamed effective onlywhen the same output pulse is obtained from both photomultiplier tubes.

However, the following problems have been associated with opticalmeasurement apparatuses that eliminate dark current pulses using themethod of simultaneous measurement. First, the apparatus is complex tomanufacture and large in size, since it requires two photomultipliertubes, two high-speed processing circuits, and simultaneous countingcircuits. Further, counting efficiency, or precision, drops becausephotons must be impinged simultaneously on both photomultiplier tubes.

Another technique and the like for detecting multiple photonssimultaneously are well known in the art. In this technique, the outputpulse is considered as valid only when the peak value of the outputpulse is greater than or equal to a specified threshold value. However,the following problems are associated with optical measurementapparatuses that eliminate dark current pulses by providing such athreshold value. As described by Mikio Yamashita, Osamu Yura, andYasushi Kawada in “Utilization of High-Gain First-Dynode PMTs forMeasuring the Average Numbers of Photoelectrons in Weak Light andScintillations” (Bulletin of the Electrotechnical Laboratory, Vol. 47,Nos. 9 and 10, 1988), for example, dark current pulses have nearlyidentical output waveforms with that of an output pulse that isgenerated when a single photoelectron is emitted from the photocathode.However, it is impossible to completely separate, based on outputwaveforms from the photomultiplier tube, an event in which a singlephotoelectron is emitted from the photocathode from another event inwhich a plurality of photoelectrons are emitted. It is necessary to seta large threshold value (for example, a value equivalent to a peak valueof an output pulse for four photoelectrons) in order to eliminate darkcurrent pulses. Hence, by setting a large threshold value, it ispossible to effectively eliminate dark current pulses, but morephotoelectrons will pass through uncounted, resulting in a drop incounting efficiency or a drop in precision.

In order to overcome this problem, optical measurement apparatuses,capable of separately detecting an event in which a single photoelectronis emitted and another event in which multiple photoelectrons areemitted, have been disclosed, for example, in Japanese unexamined patentapplication publications Nos. HEI-9-196752, HEI-9-329548, andHEI-10-227695. However, the optical measurement apparatuses disclosedtherein estimate the average number of photoelectrons and the like, butare incapable of eliminating dark current pulses.

DISCLOSURE OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide an optical measurement apparatus and a method for opticalmeasurement which is capable of eliminating dark current pulseseffectively and which is capable of measuring light with high precision.

In order to overcome the above-described problem, the present inventionprovides an optical measurement apparatus comprising: a photodetectingportion emitting photoelectrons that correspond to the amount of lightin an incident light beam and outputting an output signal thatcorresponds to the number of the photoelectrons; a comparing portioncomparing the output signal with a predetermined threshold value andoutputting a comparison result signal when the output signal is greaterthan the threshold value, the comparing portion outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue; and a measuring portion performing measurement in accordance withthe comparison result signal, wherein the threshold value is larger thanan output signal that the photodetecting portion outputs when thephotodetecting portion emits only one photoelectron, the threshold valuebeing smaller than another output signal that the photodetecting portionoutputs when the photodetecting portion emits two photoelectrons,whereby the measuring portion performs measurement when thephotodetecting portion emits two or more photoelectrons, the measuringportion failing to perform measurement when the photodetecting portionemits only one photoelectron.

According to the optical measurement apparatus of the present invention,the photodetecting portion outputs an output signal corresponding to thenumber of the photoelectrons. The apparatus outputs a comparison resultsignal only when the output signal is larger than the threshold value,which is larger than an output signal that is outputted when a singlephotoelectron is emitted and which is smaller than another output signalthat is outputted when two photoelectrons are emitted. Accordingly, itis possible to effectively eliminate dark current pulses having anoutput waveform nearly identical with that of the output pulse that isoutputted when a single photoelectron is emitted. Another output signalthat is outputted when two or more photoelectrons are emitted is noteliminated, but a comparison result signal is effectively outputted. Asa result, the optical measurement apparatus can perform measurementswith high precision.

It is preferable that the optical measurement apparatus of the presentinvention further comprises a setting portion setting the thresholdvalue.

It is noted that an output signal of a dark current pulse or an outputsignal that is outputted when a single photoelectron is emittedfluctuates due to the ambient temperature and other aspects of theworking environment. By providing the setting portion that sets thethreshold value, it is possible to appropriately reset the thresholdvalue according to the working environment. As a result, the opticalmeasurement apparatus can perform measurements with high precisionregardless of the working environment.

It is preferable that the setting portion measures an output signal thatthe photodetecting portion outputs when the photodetecting portionoutputs a single photoelectron, the setting portion setting thethreshold value within a range of the amount of the output signal andtwo times the amount of the output signal.

By measuring the output signal that is outputted when a singlephotoelectron is emitted and by setting the threshold to a value betweenone and two times the output signal, it is possible to establish anappropriate threshold value even if the output signal outputted when asingle photoelectron is emitted is not previously known. As a result,the optical measurement apparatus can perform measurements with highprecision even when the output signal for a single emitted photoelectronis not previously known.

It is preferable that the setting portion measures, for a fixed periodof time, output signals that the photodetecting portion outputs when thephotodetecting portion detects no beam of light in a dark state, thesetting portion setting the threshold value to a maximum value of theoutput signals measured during the fixed period of time.

By measuring output signals in a dark state for the fixed period of timeand by setting the maximum value of the measured output signals as thethreshold value, it is possible to set an appropriate threshold valueeven if the output signals in a dark state are not previously known. Asa result, the optical measurement apparatus can perform measurementswith high precision even when the output signal in a dark state are notpreviously known.

It is preferable that the threshold value has a value within a range of1.2 to 1.8 times an output signal that the photodetecting portionoutputs when the photodetecting portion emits a single photoelectron. Itis more preferable that the threshold value has a value within a rangeof 1.3 to 1.5 times an output signal that the photodetecting portionoutputs when the photodetecting portion emits a single photoelectron.

It is preferable that the photodetecting portion includes: aphotocathode emitting photoelectrons that correspond to the amount oflight in the incident light beam; an accelerating portion acceleratingthe photoelectrons emitted from the photocathode; and a semiconductorphotodetector receiving the photoelectrons accelerated by theaccelerating portion and outputting a signal that corresponds to thenumber of the photoelectrons.

By employing the photodetecting portion that is provided with thephotocathode, the accelerating portion, and the semiconductorphotodetector, it is possible to effectively detect separately theevent, in which a single photoelectron is emitted from the photocathodeand the other event, in which two or more photoelectrons are emittedfrom the photocathode simultaneously (or at extremely close timings). Asa result, the optical measurement apparatus can perform measurementswith extremely high efficiency.

It is preferable that the semiconductor photodetector includes anavalanche photodiode.

According to another aspect, the present invention provides ascintillation counter, comprising: a scintillator converting beta raysemitted from an object of measurement into fluorescent light; aphotodetecting portion receiving the fluorescent light, and emittingphotoelectrons that correspond to the amount of the fluorescent light,thereby outputting an output signal that corresponds to the number ofthe photoelectrons; a comparing portion comparing the output signal witha predetermined threshold value and outputting a comparison resultsignal when the output signal is greater than the threshold value, thecomparing portion outputting no comparison result signal when the outputsignal is not greater than the threshold value; and a measuring portioncounting the fluorescent light in accordance with the comparison resultsignal, wherein the threshold value is larger than an output signal thatthe photodetecting portion outputs when the photodetecting portion emitsonly one photoelectron, the threshold value being smaller than anotheroutput signal that the photodetecting portion outputs when thephotodetecting portion emits two photoelectrons, whereby the measuringportion performs counting when the photodetecting portion emits two ormore photoelectrons, the measuring portion failing to perform countingwhen the photodetecting portion emits only one photoelectron.

The scintillation counter of the present invention not only caneffectively eliminate dark current pulses, but also can effectivelyoutput a comparison result signal without eliminating such an outputsignal that is outputted when two or more photoelectrons are emitted. Asa result, it is possible to count scintillation with high precision.

According to still another aspect, the present invention provides aparticle counter, comprising: a scattered light generating portiongenerating scattered light by scattering light according to particlesmixed in a sample to be measured; a photodetecting portion receiving thescattered light, and emitting photoelectrons that correspond to theamount of the scattered light, thereby outputting an output signal thatcorresponds to the number of the photoelectrons; a comparing portioncomparing the output signal with a predetermined threshold value andoutputting a comparison result signal when the output signal is greaterthan the threshold value, the comparing portion outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue; and a measuring portion counting the particles in accordance withthe comparison result signal, wherein the threshold value is larger thanan output signal that the photodetecting portion outputs when thephotodetecting portion emits only one photoelectron, the threshold valuebeing smaller than another output signal that the photodetecting portionoutputs when the photodetecting portion emits two photoelectrons,whereby the measuring portion performs counting when the photodetectingportion emits two or more photoelectrons, the measuring portion failingto perform counting when the photodetecting portion emits only onephotoelectron.

The particle counter of the present invention not only can effectivelyeliminate dark current pulses, but also can effectively output acomparison result signal without eliminating such an output signal thatis outputted when two or more photoelectrons are emitted. As a result,it is possible to count particles with high precision.

In order to overcome the above-described problem, the present inventionprovides an optical measurement method comprising: a photodetecting stepemitting photoelectrons that correspond to the amount of light in anincident light beam and outputting an output signal that corresponds tothe number of the photoelectrons; a comparing step comparing the outputsignal with a predetermined threshold value and outputting a comparisonresult signal when the output signal is greater than the thresholdvalue, the comparing step outputting no comparison result signal whenthe output signal is not greater than the threshold value; and ameasuring step performing measurement in accordance with the comparisonresult signal, wherein the threshold value is larger than an outputsignal that the photodetecting step outputs when the photodetecting stepemits only one photoelectron, the threshold value being smaller thananother output signal that the photodetecting step outputs when thephotodetecting step emits two photoelectrons, whereby the measuring stepperforms measurement when the photodetecting step emits two or morephotoelectrons, the measuring step failing toe perform measurement whenthe photodetecting step emits only one photoelectron.

According to the optical measurement method of the present invention,the photodetecting step outputs an output signal corresponding to thenumber of the photoelectrons. A comparison result signal is outputtedonly when the output signal is larger than the threshold value, which islarger than an output signal that is outputted when a singlephotoelectron is emitted and which is smaller than another output signalthat is outputted when two photoelectrons are emitted. Accordingly, itis possible to effectively eliminate dark current pulses having anoutput waveform nearly identical with that of the output pulses that isoutputted when a single photoelectron is emitted. Another output signalthat is outputted when two or more photoelectrons are emitted is noteliminated, but a comparison result signal is effectively outputted. Asa result, the optical measurement method can perform measurements withhigh precision.

It is preferable that the optical measurement method of the presentinvention further comprises a setting step setting the threshold value.

It is noted that an output signal of a dark current pulse or an outputsignal that is outputted when a single photoelectron is emittedfluctuates due to the ambient temperature and other aspects of theworking environment. By providing the setting step that sets thethreshold value, it is possible to appropriately reset the thresholdvalue according to the working environment. As a result, the opticalmeasurement method can perform measurements with high precisionregardless of the working environment.

It is preferable that the setting step measures an output signal thatthe photodetecting step outputs when the photodetecting step outputs asingle photoelectron, the setting step setting the threshold valuewithin a range of the amount of the output signal and two times theamount of the output signal.

By measuring the output signal that is outputted when a singlephotoelectron is emitted and by setting the threshold to a value betweenone and two times the output signal, it is possible to establish anappropriate threshold value even if the output signal outputted when asingle photoelectron is emitted is not previously known. As a result,the optical measurement method can perform measurements with highprecision even when the output signal for a single emitted photoelectronis not previously known.

It is preferable that the setting step measures, for a fixed period oftime, output signals that the photodetecting step outputs when thephotodetecting step detects no beam of light in a dark state, thesetting step setting the threshold value to a maximum value of theoutput signals measured during the fixed period of time.

By measuring output signals in a dark state for the fixed period of timeand by setting the maximum value of the measured output signals as thethreshold value, it is possible to set an appropriate threshold valueeven if the output signals in a dark state are not previously known. Asa result, the optical measurement method can perform measurements withhigh precision even when the output signal in a dark state are notpreviously known.

It is preferable that the threshold value has a value within a range of1.2 to 1.8 times an output signal that the photodetecting step outputswhen the photodetecting step emits a single photoelectron. It is morepreferable that the threshold value has a value within a range of 1.3 to1.5 times an output signal that the photodetecting step outputs when thephotodetecting step emits a single photoelectron.

According to another aspect, the present invention provides ascintillation counting method, comprising: a fluorescent converting stepusing a scintillator to convert beta rays emitted from an object ofmeasurement into fluorescent light; a photodetecting step receiving thefluorescent light, and emitting photoelectrons that correspond to theamount of the fluorescent light, thereby outputting an output signalthat corresponds to the number of the photoelectrons; a comparing stepcomparing the output signal with a predetermined threshold value andoutputting a comparison result signal when the output signal is greaterthan the threshold value, the comparing step outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue; and a measuring step counting the fluorescent light in accordancewith the comparison result signal; wherein the threshold value is largerthan an output signal that the photodetecting step outputs when thephotodetecting step emits only one photoelectron, the threshold valuebeing smaller than another output signal that the photodetecting stepoutputs when the photodetecting step emits two photoelectrons, wherebythe measuring step performs counting when the photodetecting step emitstwo photoelectrons, the measuring step failing to perform counting whenthe photodetecting step emits only one photoelectron.

The scintillation counting method of the present invention not only caneffectively eliminate dark current pulses, but also can effectivelyoutput a comparison result signal without eliminating such an outputsignal that is outputted when two or more photoelectrons are emitted. Asa result, it is possible to count scintillation with high precision.

According to still another aspect, the present invention provides aparticle counting method, comprising: a scattered light generating stopgenerating scattered light by scattering light according to particlesmixed in a sample to be measured; a photodetecting step receiving thescattered light, and emitting photoelectrons that correspond to theamount of the scattered light, thereby outputting an output signal thatcorresponds to the number of the photoelectrons; a comparing stepcomparing the output signal with a predetermined threshold value andoutputting a comparison result signal when the output signal is greaterthan the threshold value, the comparing step outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue; and a measuring step counting the particles in accordance withthe comparison result signal, wherein the threshold value is larger thanan output signal that the photodetecting step outputs when thephotodetecting step emits only one photoelectron, the threshold valuebeing smaller than another output signal that the photodetecting stepoutputs when the photodetecting step emits two photoelectrons, wherebythe measuring step performs counting when the photodetecting step emitstwo or more photoelectrons, the measuring step failing to performcounting when the photodetecting step emits only one photoelectron.

The particle counting method of the present invention not only caneffectively eliminate dark current pulses, but also can effectivelyoutput a comparison result signal without eliminating such an outputsignal that is outputted when two or more photoelectrons are emitted. Asa result, it is possible to count particles with high precision.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows the construction of a liquid scintillation counter servingas an optical measurement apparatus according to a first embodiment ofthe present invention;

FIG. 2 shows the distribution of wave heights outputted from the HPD 24in FIG. 1;

FIG. 3 shows the distribution of wave heights outputted from the HPD 24in FIG. 1 during a dark state;

FIG. 4(a) is a flowchart showing the operations of the liquidscintillation counter in FIG. 1;

FIG. 4(b) is a flowchart showing the process for setting the thresholdvalue in FIG. 4(a);

FIG. 4(c) is a flowchart showing the measurement process in FIG. 4(a);

FIG. 5 is a graph comparing the counting efficiencies of the liquidscintillation counter of FIG. 1 with counting efficiency of acomparative example;

FIG. 6 shows the construction of a particle counter serving as anoptical measurement apparatus according to a second embodiment of thepresent invention; and

FIG. 7 shows the content displayed by the particle counter of FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

An optical measurement apparatus according to preferred embodiments ofthe present invention will be described while referring to FIGS. 1-7.

First, an optical measurement apparatus according to a first embodimentof the present invention will be described with reference to FIGS. 1-5.The optical measurement apparatus according to the present embodiment isa liquid scintillation counter.

The liquid scintillation counter according to the present embodiment isfor detecting the composition and the like of an object to be measured(hereinafter referred to as “sample”) by converting β rays emitted fromthe sample into light by a liquid scintillation and counting thescintillation light (fluorescent light). The optical measurementapparatus of the present embodiment is included in the scintillationcounter according to the present embodiment. First, the construction ofthe liquid scintillation counter according to the present embodimentwill be described. FIG. 1 shows the construction of the liquidscintillation counter according to the present embodiment.

The liquid scintillation counter 10 according to the present embodimentincludes a sample chamber 12 and a measurement chamber 14. The samplechamber 12 is provided with a plurality of vials 20 in which a sample 16and a liquid scintillator 18 are introduced, and a rotating base 22 thatis rotatably provided for supporting the plurality of vials.

The liquid scintillator 18 can be, for example, a solution of2,5-diphenyloxazole dissolved in toluene. When β rays are emitted fromthe sample in this case, fluorescent light having a wavelength of 380 nmis emitted from the liquid scintillator 18. When tritium is used as thesample, this fluorescent light is a pulse light having an average ofabout 60 photons.

The measurement chamber 14 is provided with a hybrid photodetector(hereinafter referred to as an HPD 24), a charge amplifier 26, a voltageamplifier 28, a comparator 30, a counter 32, a multichannel analyzer 34,a CPU 36, a display 38, and a switch 140. Here, the HPD 24, chargeamplifier 26, and voltage amplifier 28 make up the photodetecting unit.

Next, each component in the liquid scintillation counter will bedescribed in greater detail.

The HPD 24 is an electron tube having a photocathode 24 a for emittingphotoelectrons corresponding to the number of incident photons and anavalanche photodiode (hereinafter referred to as an APD 24 b), which isa semiconductor photodetector for outputting a signal corresponding tothe number of photoelectrons emitted from the photocathode 24 a. Thephotocathode 24 a and APD 24 b are disposed opposing each other in avacuum chamber 24 c. A high-voltage source 24 d applies a high negativevoltage (for example, −8 kV) to the photocathode 24 a, while a biascircuit 24 e applies a reverse bias voltage (for example, −150 V) acrossan anode and a cathode of the APD 24 b. An electronic lens unit notshown in the diagram is also provided in the HPD 24, enabling thephotoelectrons emitted from the photocathode 24 a to be efficientlyimpinged on the APD 24 b.

The HPD 24 is positioned such that the photocathode 24 a faces thesample chamber 12 in order to receive (a portion of) the fluorescentlight emitted from the liquid scintillator 18. More specifically, ashutter 40 is provided in the wall separating the sample chamber 12 andmeasurement chamber 14 and positioned in opposition to the photocathode24 a. Opening the shutter 40 during measurements allows (a portion of)fluorescent light emitted from the liquid scintillator 18 to fallincident on the photocathode 24 a. The light emitted from the liquidscintillator 18 is blocked when not performing measurements by closingthe shutter 40.

When photons impinge on the photocathode 24 a, photoelectronscorresponding to the number of the incident photons are emitted from thephotocathode 24 a. These photoelectrons are accelerated by the work ofan electric field, converged by the electronic lens unit, and impingedon the APD 24 b. When the photoelectrons enter the APD 24 b, numeroushole-electron pairs are generated when they lose energy. This determinesthe multiplication factor of the first stage. The multiplication factorof the first stage is dependent on the acceleration voltage of theelectrons (the voltage applied to the photocathode), and isapproximately 1,200 when the voltage is −8 kV. The electrons are furtheramplified through avalanche multiplication to about 50 times, resultingin a gain of approximately 60,000 times through the effects of theentire APD 24 b. Since the multiplication factor of approximately 1,200in the first stage is extremely large, the multiplication fluctuationsin the HPD 24 that uses the APD 24 b are extremely small. Accordingly,the distribution of outputted wave heights shown in FIG. 2 are obtainedfrom the HPD 24 when the HPD 24 receives multiple photons. With this HPD24, it is possible to detect how many photoelectrons are emitted fromthe photocathode 24 a. The results shown in FIG. 2 were obtained whenapplying a voltage of −8 kV to the photocathode 24 a and a reverse biasvoltage of −150 V to the APD 24 b and conducting measurements using anOltec model no. 142A preamplifier.

As the HPD 24, the electron tubes disclosed in Japanese unexaminedpatent application publications Nos. HEI-9-312145 and HEI-6-318447, aphotomultiplier tube disclosed in Japanese unexamined patent applicationpublication No. HEI-8-148113, and an electron tube disclosed in Japaneseunexamined patent application publication No. HEI-9-297055 can be widelyused.

As shown in FIG. 1, the charge amplifier 26 includes an operationalamplifier 26 a, a capacitor 26 b connected to a minus input terminal andan output terminal of the operational amplifier 26 a, and a resistor 26c connected in parallel to the capacitor 26 b. The signal outputted fromthe APD 24 b is inputted into the minus input terminal of theoperational amplifier 26 a, while the plus input terminal of theoperational amplifier 26 a is grounded. The charge amplifier 26integrates the charge amount received from the HPD 24, and outputs theintegrated result as the voltage. For example, if the capacitance of thecapacitor 26 b is 1 pF and the resistance of the resistor 26 c is 1 GΩ,then the charge amplifier 26 outputs an inputted charge amount of 1 pCas a voltage of 1 V. In this case, the time constant of the outputtedvoltage waveform is 1 ms.

The voltage amplifier 28 amplifies and outputs the voltage received fromthe charge amplifier 26. The amplifying gain is set to 50 times, forexample.

The switch 140 switches, in response to instructions from the CPU 36, anoutput destination to output the output signal received from the voltageamplifier 28 to either the comparator 30 or the multichannel analyzer34.

The comparator 30 compares the output signal outputted from the voltageamplifier 28 to a prescribed threshold value and outputs an outputsignal only when the output signal is greater than the threshold value.More specifically, the output signal received from the voltage amplifier28 is inputted into the plus input terminal of the comparator 30, whilea reference voltage indicative of the threshold value is inputted intothe minus input terminal. The threshold value is set larger than anoutput signal that will be received from the voltage amplifier 28 when asingle photoelectron is emitted from the photocathode 24 a and smallerthan another output signal that will be outputted from the voltageamplifier 28 when two photoelectrons are emitted from the photocathode24 a. Taking into account the gain of the APD 24 b (60,000 times), theconversion gain of the charge amplifier (1 V/1 pC), and the gain of thevoltage amplifier 28 (50 times), the output signal will be 0.5 V whenone photoelectron is emitted and 1.0 V when two photoelectrons areemitted. Hence, the threshold value is set between 0.5 and 1.0 V. It isnoted that the CPU 36 determines the threshold value.

The counter 32 counts the pulses outputted from the comparator 30 andoutputs the result to the CPU 32. The display 38 displays the countedvalue determined by the counter 32, and a detected composition and thelike of the sample.

The multichannel analyzer 34 measures the distribution of wave heightsof output signals outputted from the voltage amplifier 28. That is, eachtime a pulse output signal is outputted from the voltage amplifier 28,the multichannel analyzer 34 adds and accumulates a prescribed value(such as 1) to a channel (address) corresponding to the wave height(voltage) of the outputted signal pulse. In this way, the multichannelanalyzer 34 measures the distribution of wave heights for output signalsreceived from the voltage amplifier 28.

The CPU 36 controls the entire apparatus. More specifically, the CPU 36controls the switch 140, controls the content displayed on the display38, determines the threshold value for the comparator 30, controlsopening and closing of the shutter 40, and the like. The method fordetermining the threshold value is described later in more detail. TheCPU 36 further controls rotation of the rotating base 22 in the samplechamber 12, moving the vial 20 to be measured to a positioncorresponding to the HPD 24. The various control processes of the CPU 36given above will be described with reference to the flowcharts shown inFIGS. 4(a)-4(c). Program data implementing the processes in theseflowcharts is stored, for example, in a ROM not shown in the drawings.

Next, the method for determining the threshold value performed by theCPU 36 will be described in more detail. First, the CPU 36 closes theshutter 40 and switches the switch 140 to the multichannel analyzer 34side. By closing the shutter 40, the CPU 36 creates a state of noincident light on the HPD 24 (hereinafter referred to as a “darkstate”). Since photoelectrons are discharged discretely from thephotocathode 24 a one electron at a time in a dark state, the voltageamplifier 28 outputs in a pulse form output signals of wave heightscorresponding to one electron worth. Accordingly, it is possible todetermine an output signal that will be outputted from the voltageamplifier 28 when a single photoelectron is emitted from thephotocathode 24 a by measuring with the multichannel analyzer 34 thedistribution of wave heights in a dark state.

In other words, each time a pulse output signal is outputted from thevoltage amplifier 28, the multichannel analyzer 34 adds and accumulatesa prescribed value (for example, 1) to the channel (address)corresponding to the wave height (voltage) of the output signal pulse.In this way, the multichannel analyzer 34 measures the wave heightdistribution for signals outputted from the voltage amplifier 28. Hence,the output wave height distribution in a dark state is obtained after aprescribed period of time, as shown in FIG. 3. The horizontal axis inFIG. 3 represents the channel corresponding to the wave height of theoutput signal pulse outputted from the voltage amplifier 28. In thisexample, the 1,200^(th) channel on the horizontal axis corresponds to awave height value of 0.56 V of the output signal pulse. The verticalaxis represents a relative value for the number of times (count) that anoutput signal has been obtained for each wave height. Hence, the valueof the output wave height, at which the wave height distribution peaks(hereinafter referred to as the “distribution peak wave height”), isequivalent to an average output signal that is outputted from thevoltage amplifier 28 when a single photoelectron is emitted from thephotocathode 24 a. The output signal outputted from the voltageamplifier 28 when two photoelectrons are emitted from the photocathode24 a can be determined by doubling the output signal that is outputtedwhen a single photoelectron is emitted.

The following is a description of how the threshold value is set withina range greater than an output signal that is outputted when a singlephotoelectron is emitted and smaller than another output signal that isoutputted when two photoelectrons are emitted.

Varying the voltage applied to the photocathode 24 a and the reversebias voltage applied to the APD 24 b changes the output wave heightdistribution (FIG. 3) of the output signals outputted from the voltageamplifier 28. This also changes the maximum output wave height, which isthe largest value from among the obtained output wave heights; that is,the maximum value of the dark current pulse. The inventors of thepresent invention varies the voltage applied to the photocathode 24 a at−7.5 kV, −8.0 kV, and −8.5 kV; varied the reverse bias voltage of theAPD 24 b at −148 V, −150 V, and −152 V; and measured the output waveheight for a fixed period of time for each case to obtain the maximumvalue of the acquired output wave heights (that is, the maximum value ofthe dark current pulse). The results of these measurements are listed inTable 1 below. It is noted that when preparing Table 1, the maximumvalue of the dark current pulse is converted to the number ofphotoelectrons. In other words, the maximum output wave height fromamong the wave height distribution obtained for each measurement isdivided by the distribution peak wave height, with the result indicatingthe maximum value of the dark current pulse.

TABLE 1 APD reverse bias voltage (V) −148 −150 −152 Photocathode −7.51.26 1.26 1.29 voltage (kV) −8.0 1.34 1.23 1.21 −8.5 1.28 1.18 1.28

According to the results shown in Table 1, the threshold value should beset greater than 1.2 times the output signal that is outputted when asingle photoelectron is emitted, but not too large, in order toeffectively eliminate the dark current and, moreover, to effectivelyoutput another output signal that is outputted when two or moreelectrons are emitted. That is, it is preferable that the thresholdvalue be set within the range 1.2-1.8 times the output signal that isoutputted when a single photoelectron is emitted, and even morepreferable within the range 1.3-1.5 times the output signal that isoutputted when a single photoelectron is emitted. Therefore, we can seethat the CPU 36 should set the threshold value to a single value withinthe range of 1.2-1.8 times (and more preferably 1.3-1.5 times) theoutput signal of the voltage amplifier 28 that corresponds to thedistribution peak wave height in a dark state.

In the method described above, the distribution peak wave height isdetermined in a dark state and the threshold value is set within therange 1.2-1.8 times the output signal corresponding to that value. It isnoted, however, that the maximum wave height can be determined bymeasuring, for a prescribed time period, the output wave heightdistribution in a dark state, as in the experiment described above.Accordingly, the threshold value can also be set to this maximum waveheight value or to a value slightly larger than the maximum wave heightvalue.

Next, a description will be given for the operations of the liquidscintillation counter 10 according to the present embodiment, along witha description of a method for scintillation counting according to thepresent embodiment, while referring to the flowcharts in FIGS.4(a)-4(c).

FIG. 4(a) shows the process for analyzing a sample using the liquidscintillation counter 10 according to the present embodiment. In S100,the CPU determines the threshold value (reference voltage) of thecomparator 30. In S200, the sample is actually measured and analyzed. Inthis way, an accurate measurement can be performed by setting thereference voltage of the comparator 30 each time prior to beginning ameasurement because the gain of the HPD 24, the voltage from the powersource, and the like change slightly due to the ambient temperature andother operating conditions. However, it is unnecessary to perform thethreshold value determining process of S100 each time prior toperforming measurements in S200. For example, it is possible to performthis step only when the user deems it necessary.

The process for determining the threshold value for the comparator 30(S100) includes the following steps shown in FIG. 4(b).

A high negative voltage (−8 kV) is applied to the photocathode 24 awhile a reverse bias voltage is applied to the APD 24 b. In this state,the shutter 40 is closed in S110 to create a dark state. In S120, theswitch 140 is switched to the multichannel analyzer 34 side and theoutput wave height distribution in the dark state is measured for theprescribed time period.

The output wave height distribution is similar to that shown in FIG. 3.It is possible to see from the distribution peak wave height (1200^(th)channel) that the average output signal outputted from the voltageamplifier 28 when a single photoelectron is emitted from thephotocathode 24 a is 0.56 V (0.50 V as a calculated value). In S130, theCPU 36 obtains the output wave height distribution data from themultichannel analyzer 34, determines the distribution peak wave height(in this case, the 1200^(th) channel), and determines the magnitude ofthe corresponding output signal from the voltage amplifier 28 (in thiscase, 0.50 V). In S140, the CPU 36 determines the threshold value to be0.78 V, for example, equivalent to 1.4 times the magnitude of the outputsignal (the magnitude of an output signal that is outputted when asingle photoelectron is emitted), and sets the reference voltage of thecomparator 30 to this threshold value.

After setting the reference voltage in this way (S140), the CPU 36toggles the switch 140 to the comparator 30 side in S150, and theprocess for determining the threshold value (S100) ends.

It is noted that the spread in the output signals that are obtained whena single photoelectron is emitted, as seen in FIG. 3, is caused byfluctuation in amplification by the HPD 24 and noise from the chargeamplifier 26 and the like. However, since the amplification fluctuationby the HPD 24 is extremely small compared to that by photomultipliertubes, it is possible to set an appropriate threshold value.

Next, in the measurement process (S200), the sample 16 and liquidscintillator 18 are set in a manner described below.

An example of the sample 16 is the product of an antigen-antibodyreaction between a hormone, tumor marker, or other antigen and anantibody marked by tritium, which is the source of the β rays. It thisexample, a large amount of emitted β rays indicates the existence of alarge amount of antigens, enabling the detection of hormonalabnormality, the existence of a tumor, or the like. After the sample 16is introduced into a vial 20, the vial 20 is filled with the liquidscintillator 18.

After a sensor or the like not shown in the drawings detects that a vial10 filled with the sample 16 and liquid scintillator 18 has been placedon the rotating base 22, then the CPU 36 begins in S210 the preliminaryprocess for measurement, as shown in FIG. 4(c). In other words, the CPU36 rotates the rotating base 22 until the vial 20 reaches the measuringposition, that is, a position opposing the HPD 24.

When the vial 20 reaches the measuring position, the CPU 36 opens theshutter 40 in S220 and begins measurements in S230.

In the measuring process (S230), (a portion of) fluorescent lightemitted from the liquid scintillator 18 impinges in pulses on thephotocathode 24 a. When photons impinge on the photocathode 24 a,photoelectrons corresponding to the number of impinged photons areemitted from the photocathode 24 a. The photoelectrons are acceleratedby the effects of an electric field, converged by the effect of theelectronic lens unit, and impinged on the APD 24 b. The photoelectronsimpinged on the APD 24 b generate multiple hole-electron pairs when theylose their energy and are further amplified through avalanchemultiplication before being outputted.

A pulse charge outputted from the HPD 24 in this way is amplified by thecharge amplifier 24 and outputted as a voltage. The voltage, is furtheramplified by the voltage amplifier 28. The output signal pulse outputtedfrom the voltage amplifier 28 is inputted into the comparator 30. Thecomparator 30 compares the output signal pulse to a reference voltageand outputs a single logical pulse signal when the wave height of theoutput signal pulse is higher than the reference voltage. Since thereference voltage for the comparator 30 is met to 0.78 V, whichcorresponds to 1.4 times the output signal that is outputted when asingle photoelectron is emitted, a logic pulse signal is not outputtedby the comparator 30 in response to a dark current pulse correspondingto an output signal that is outputted when a single photoelectron isemitted. On the other hand, a logic pulse signal is effectivelyoutputted from the comparator 30 in response to an output signal that isoutputted when two or more photoelectrons are emitted. Hence, when anoutput signal pulse based on two or more photoelectrons is inputted, thecomparator 30 outputs a single logic pulse signal indicating one event,regardless of the amount of photoelectrons.

While it is conceivable that there will be cases in which a singlephotoelectron is emitted due to fluorescent light from the liquidscintillator 18, it is more likely that a plurality of photons will beemitted in one event when performing radiation detection or the likeusing the liquid scintillator 18. It is not particularly problematic toeliminate, along with the dark current pulses, output signals that areoutputted when a single photoelectron is emitted.

The counter 32 counts output signals outputted from the comparator 30.In other words, the counter 32 measures the number of events by countingpulse signals from the comparator 30. This count value is inputted intothe CPU 36. Based on this count value, the CPU 36 determines theexistence of hormonal abnormalities or tumors and displays the resultson the display 38.

As described above, the liquid scintillation counter 10 of the presentembodiment includes the HPD 24, charge amplifier 26, voltage amplifier28, comparator 30, counter 32, multichannel analyzer 34, CPU 36, display38, and the like. The HPD 24 has the photocathode 24 a and APD 24 b foroutputting signals in response to the number of incident photons. Thecomparator 30 compares an output signal outputted from the HPD 24 andamplified by the charge amplifier 26 and voltage amplifier 20 with athreshold value. Here, the threshold value is set larger than an outputsignal that is outputted when a single photoelectron is emitted from thephotocathode 24 a and smaller than another output signal that isoutputted when two photoelectrons are emitted. The comparator 30 outputsa logic pulse signal serving as a comparison result signal only when theoutput signal is larger than the threshold value.

Next, the operations and effects of the liquid scintillation counteraccording to the present embodiment will be described. According to theliquid scintillation counter 10 of the present embodiment, the HPD 24outputs a signal that corresponds to the number of the emittedphotoelectrons. Further, the comparator 30 outputs only those outputsignals, larger than a threshold value equivalent to 1.4 times, anoutput signal that is outputted when a single photoelectron is emitted.Hence, the liquid scintillation counter can effectively eliminate darkcurrent pulses having an output waveform substantially identical to thatof an output signal that is outputted when a single photoelectron isemitted, without eliminating but effectively outputting those outputsignals that are outputted when two or more photoelectrons are emitted.As a result, high-precision measurements are possible.

Although the frequency in which photoelectrons forming dark currentpulses are emitted from the photocathode depends on the size, type, andthe like of the photocathode, in general this frequency is several tensto several thousands counts per second. Therefore, these dark currentpulses can create much noise when detecting weak light. It becomesnecessary to decrease the counting efficiency is order to remove thislarge noise. To overcome this, the liquid scintillation counter 10 ofthe present embodiment effectively removes dark current pulses, therebyimproving counting efficiency.

For comparison purposes, FIG. 5 shows the counting efficiencies of aconventional liquid scintillation counter employing a simultaneouscounting method and of the liquid scintillation counter 10 of thepresent embodiment. The horizontal axis represents the average number ofphotons emitted in a single event, while the vertical axis representsthe counting efficiency. According to the conventional liquidscintillation counter employing the simultaneous measuring method, theratio between the numbers of photons impinging on the twophotomultiplier tubes is frequently about 3:7. Therefore, the countingefficiency for the optical measurement apparatus according to thesimultaneous counting method was set to the percentage, at which lightwas detected simultaneously by both photomultiplier tubes when light wasguided at this ratio into the photomultiplier tubes. In the liquidscintillation counter 10 according to the present embodiment, twocounting efficiencies were set to the percentages: a rate, at which twoor more photoelectrons were detected when 70% of scintillation light wasincident on the HPD 24 (70% light using rate); and another rate, atwhich two or more photoelectrons were detected when 85% of thescintillation light was incident on the HPD 24 (85% light using rate).For example, when an average of three photons are emitted in a singleevent, the light using efficiency of the optical measurement apparatusemploying the simultaneous counting method is about 51%, while thecounting efficiency of the liquid scintillation counter 10 according tothe present embodiment is 62% (at 70% light using rate) or 72% (at 85%light using rate). Hence, the light using efficiency of the opticalmeasurement apparatus according to the present embodiment is improvedover the optical measurement apparatus using the simultaneous countingmethod. For simplicity, the quantum efficiency of the photocathode 24 ahas been set at 100%.

Since the liquid scintillation counter 10 of the present embodiment doesnot require two photomultiplier tubes, two high-speed processingcircuits, simultaneous counting circuits, and the like, this device canbe simplified and made more compact than a liquid scintillation counterusing the simultaneous counting method. Eliminating the need forsimultaneous counting and the like relaxes the necessity for high-speedprocessing.

By having the ability to set a reference voltage (threshold value) ofthe comparator 30 via the CPU 36, the liquid scintillation counter 30 ofthe present embodiment can reset the threshold value to an appropriatevalue for the working environment. As a result, measurements can beperformed with high accuracy, regardless of the operating environment.

The liquid scintillation counter according to the present embodimentmeasures output signals for a single photoelectron using themultichannel analyzer 34 and sets the threshold value at 1-2 times (morespecifically, 1.4 times) this output signal with the comparator 30.Accordingly, it is possible to set an appropriate threshold value evenwhen the output signal that is outputted when a single photoelectron isemitted is previously unknown.

By detecting light with the HPD 24 having the APD 24 b, the liquidscintillation counter 10 of the present embodiment can effectivelydifferentiate when a single photoelectron is emitted from thephotocathode 24 a and when two or more photoelectrons ere emitted. As aresult, dark current pulses having an output waveform substantiallyidentical to that whet a single photoelectron is emitted can be removedwith extreme effectiveness.

Next, an optical measurement apparatus according to a second embodimentof the present invention will be described with reference to FIGS. 6 and7. The optical measurement apparatus according to the present embodimentis a particle counter.

The particle counter according to the present embodiment irradiateslight on a sample such as water or air and detects scattered light thathas been scattered due to foreign matter mixed in the sample, to therebymeasure the particle size, and number of the foreign matter. Parts andcomponents in the particle counter according to the present embodimentthat art identical to those of the liquid scintillation counter of thefirst embodiment are designated by the same reference numerals to avoidduplicating description. First, the construction of the particle counteraccording to the present embodiment will be described. FIG. 6 shows theconstruction of the particle counter according to the preventembodiment.

A particle counter 50 according to the present embodiment includes asample chamber 52, a light source chamber 54, and a measurement chamber56. The sample chamber 52 includes a capillary tube 60 in which a sample58 flows, and a curved mirror 64 for gathering scattered light fromforeign matter 62 contained in the sample 58 to fall incident on themeasurement chamber 56. The light source chamber 54 includes a lightsource 66 for emitting light to irradiate the sample 58, and acondensing lens 68 for condensing the light irradiated from the lightsource 66.

The measurement chamber 56 is provided with the HPD 24, an I/Vconversion amplifier 70, a pain regulating amplifier 72, the comparator30, an A/D (analog-to-digital) converter 74, a multichannel counter 76,a peak holding circuit 78, the CPU 36, the display 38, and the switch140. Here, the HPD 24 and the I/V conversion amplifier 70 make up thelight detecting unit.

The shutter 40 is provided in a wall separating the sample chamber 52and measurement chamber 56 at a position that confronts the photocathode24 a of the HPD 24. By opening the shutter 40 during measurements, (aportion of) scattered light generated by foreign matter in the sample 58falls incident on the photocathode 24 a. When not performingmeasurements, the shutter 40 is closed to block the scattered light.

Next, each component of the particle counter will be described in moredetail.

The I/V conversion amplifier 70 converts the electric current outputtedfrom the HPD 24 to a voltage and outputs this voltage. The I/V,conversion amplifier 70 is useful for detecting light that fallsincident thereon at a high frequency, due to having a better responsethan a charge amplifier or the like.

According to instructions from the CPC 36, the switch 140 can switch theoutput destination for output signals outputted from the I/V conversionamplifier 70 to either the comparator 30 and the like or the peakholding circuit 78.

The comparator 30 compares the signals outputted from the I/V conversionamplifier 70 to a prescribed threshold value and outputs a triggersignal to the A/D converter 74 when the output signal is larger than thethreshold value. This threshold value is set larger than an outputsignal that is outputted from the I/V conversion amplifier 70 when asingle photoelectron is emitted from the photocathode 24 a and smallerthan another output signal that is outputted from the I/V conversionamplifier 70 when two photoelectrons are emitted from the photocathode24 a.

The A/D converter 74, upon receipt of the trigger signal from thecomparator 30, converts the output signal outputted from the I/Vconversion amplifier 70 from analog to digital, and outputs theresulting signal to the multichannel counter 76. The resolution of theA/D converter 74 is adjusted in order to output, as a digital value, thenumber of photoelectrons emitted from the photocathode 24 a. In otherwords, the A/D converter 74 outputs a digital value “2” when twophotoelectrons are emitted from the photocathode 24 a, a digital value“3” when three photoelectrons are emitted, and a digital value “4” whenfour photoelectrons are emitted. However, since the A/D converter 74performs A/D conversion after receiving a trigger signal from thecomparator 30 and since the threshold value of the compactor 30 is setlarger than the output signal that is outputted from the I/V conversionamplifier 70 when a single photoelectron is emitted from thephotocathode 24 a, then no signal will be outputted from the A/Dconverter 74 when a single photoelectron is emitted from thephotocathode 24 a. The gain regulating amplifier 72 adjusts the gain ofthe A/D converter 74 according to instructions from the CPU 36.

The multichannel counter 76 counts the number of digital signalsoutputted from the A/D converter 74 for each digital value and outputsthe count to the CPU 36. In other wards, each time a digital signal isoutputted from the A/D converter 74, the multichannel counter 76 addsand accumulates a prescribed value (for example, 1) at an addresscorresponding to the digital output value. Through use of themultichannel counter 76, therefore, the distribution of the number ofphotoelectrons emitted from the photocathode 24 a (that corresponds tothe distribution of the number of incident photons) is obtained.

The peak holding circuit 78 holds the peak value of output signalsoutputted from the I/V conversion amplifier 70 and outputs this value tothe CPU 36. Accordingly, it is possible to measure the maximum value ofdark current pulses within a fixed time period by toggling the switch140 to the peak holding circuit 78 side during a dark state.

Similar to the CPU 36 of the liquid scintillation counter 10 in thefirst embodiment, the CPU 36 in the present embodiment controls overalloperations of the particle counter 50, as shown in the flowcharts ofFIGS 4(a-4 c).

Next, the operations of the particle counter according to the presentembodiment will be described together with a method for countingparticles according to the present embodiment.

In order to analyze the sample 60 using the particle counter 50 of thepresent embodiment, as in the first embodiment, the threshold value ofthe comparator 30 is determined in S100 (see FIG. 4(a)). The steps inthe process for determining the threshold value are described below withreference to FIG. 4(b). That is, in S110, the shutter 40 is closed toform a dark state. In S120, the switch 140 is toggled to the peakholding circuit 76 side. In S130, the dark current pulses are measuredfor the fixed time period. In S140, the CPU 36 receives the output valuefrom the peak holding circuit 78, that is, the maximum value of theoutput signals from the I/V conversion amplifier 70 within the set timeperiod (the wave height of the maximum output shown in FIG. 3) and setsthe threshold value to this maximum value.

After setting the threshold value in S140, the CPU 36 toggles the switch140 to the comparator 30 side (S150) and begins the measurement processof S200.

In the measurement process of S200, as shown in FIG. 4(c), preparationsfor; measurements are performed in S210. That is, the sample 58, such aswater or the like, is started being introduced into the capillary tube60. The switch of the light source 66 is turned on to begin irradiatinglight on the sample 58. In S220, the shutter 40 is opened. When thesample 56 contains no foreign matter 62, then light irradiated from thelight source 66 passes through the capillary tube 60 and the sample 58and does not fall incident on the HPD 24. However, when the sample 58contains foreign matter 62, then light irradiated from the light source66 is scattered by the foreign matter 62 and a portion of the lightfalls incident on the HPD 24.

When light fells incident on the HPD 24, photoelectrons are emitted fromthe photocathode 24 a. These photoelectrons are multiplied and outputtedby the APD 24 b. The output current is converted to a voltage by the I/Vconversion amplifier 70 and converted from analog to digital by the A/Dconverter 74. Subsequently, the multichannel counter 76 counts thedigital value. Since the reference voltage of the comparator 30 is setto the maximum value of the dark current pulse (equivalent to 1.3-1.5times an output signal that is outputted when a single photoelectron isemitted), the dark current pulses equivalent to the output signal thatis outputted when a single photoelectron is emitted are not convertedfrom analog to digital and accordingly are not outputted from the A/Dconverter 74. However, output signals that are outputted when two ormore photoelectrons are emitted are converted from analog to digital andeffectively outputted from the A/D converter 74.

Signals outputted from the A/D converter 74 are taunted for each outputvalue by the multichannel counter 76. The distribution of the number ofphotoelectrons (equivalent to the distribution of the number of incidentphotons) is obtained and outputted to the CPU 36. Since the number ofincident photons is dependent on the particle site of the foreign matter62, the CPU 36 calculates the distribution of particle sizes of theforeign matter 62 based an the distribution of the number of incidentphotons and displays the distribution in the display 38, as shown inFIG. 7.

Next, the operations and effects of the particle counter according tothe present embodiment will be described. According to the particlecounter 50 of the present embodiment, the HPD 24 outputs an outputsignal that corresponds to the number of generated photoelectrons. TheA/D converter 74 outputs a signal when a trigger signal is received fromthe comparator 30. Accordingly, it is possible to effectively remove thedark current pulses having an output waveform substantially identical tothat of an output signal that is outputted when a single photoelectronis emitted, while output signals that are outputted when two or morephotoelectrons are emitted are not eliminated and can be effectivelyoutputted, thereby making it possible to perform measurements with highprecision.

If a GaAs photocathode is used in the particle counter to increasequantum efficiency and the surface area of the photocathode is enlargedto increase light using efficiency, then the frequency, in whichphotoelectrons forming a dark currant pulse are emitted from thephotocathode, becomes several tens of thousands counts per second. If ittakes about 1 μs to perform A/D conversion, then the time for measuringthe dark current pulse will take up about 10of the overall time, greatlyincreasing the likelihood that scattered light to be measured will gouncounted. Contrarily, the particle counter 50 of the present embodimentdoes not perform A/D conversion on dark current pulses but only onoutput signals due to scattered light (signal light), enabling moreefficient measurements with high precision. Therefore, the particlecounter 50 of the present embodiment can accurately detect the numberand particle size of foreign matter (dust and other contaminants)contained in the air or pure water in clean room designed formanufacturing semiconductor equipment, for example, and is extremelyuseful for controlling these environments.

Since the particle counter 50 of the present embodiment does not requiretwo photomultiplier tubes, two nigh-speed processing circuits,simultaneous counting circuits, and the like, this device can besimplified and made more compact than a particle counter using thesimultaneous counting method. Eliminating the need for simultaneouscounting and the like relaxes the necessity for high-speed processing.

By having the ability to set a reference voltage (threshold value) ofthe comparator 30 via the CPU 36, the particle counter 50 of the presentembodiment can reset the threshold value to an appropriate value for theworking environment. As a result, measurements can be performed withhigh accuracy, regardless of the operating environment.

The particle counter 50 of the present embodiment measures the maximumvalue of dark current pulses within a fixed time period using the peakholding circuit 78 and sets the threshold value to this maximum value,thereby enabling the threshold value to be set to an appropriate valueeven when the magnitude of the dark current pulse is not known inadvance.

By detecting light with the HPD 24 having the APD 24 b, the particlecounter 50 of the present embodiment can effectively differentiate whena single photoelectron is emitted from the photocathode 24 a and whentwo or more photoelectrons are emitted. As a result, dark current pulseshaving an output waveform substantially identical to that of an outputsignal that is outputted when a single photoelectron is emitted can beremoved with extreme effectiveness.

The optical measurement apparatus and method for optical measurement ofthe present invention are not limited to the embodiments describedabove. Many modifications and variations may be made therein.

For example, while the liquid scintillation counter 10 and the particlecounter 50 according to the first and second embodiments employ the APD24 b as the semiconductor light detector, a photodiode or the like canalso be used.

Further, the CPU 36 in the liquid scintillation counter 10 and theparticle counter 50 of the first and second embodiments performs theprocess to determine a threshold value in S100 in order to determine andset the threshold value. However, the threshold value can also be setmanually using a control knob or the like. A portion of the process todetermine the threshold value of S100, for example, the step for closingthe shutter 40 (S110), the steps for switching the switch 140 (S120,S150), or the like can be performed manually. Similarly, a portion ofsteps in the measurement process of S200, such as the measurementpreparation step (S210), the step to open the switch 140 (S220), or thelike can also be performed manually. Further, while the liquidscintillator 18 is used in the first embodiment described above, a solidscintillator may also be used.

Further, the above first and second embodiments are related toscintillation counting using the liquid scintillation counter 10 andparticle counting using the particle counter 50. However, the opticalmeasurement apparatus and method for optical measurement of the presentinvention are not limited to these processes. For example, the presentinvention can measure various light by controlling the measurementchamber 14 of the liquid scintillation counter 10 or the measurementchamber 56 of the particle counter 50 to measure a desired light.

In the liquid scintillation counter 10 of the first embodiment describedabove, the comparator 30 outputs a logic pulse signal as a comparisonresult signal when the output signal from the voltage amplifier 28 islarger than the reference voltage. However, the comparator 30 may outputthe output signal pulse from the voltage amplifier 26 itself instead.

INDUSTRIAL APPLICABILITY

The optical measurement apparatus and method for optical measurementaccording to the present invention can be used for measuring pulse lightwith good accuracy and can be used in a wide range of applications,including the medical field, environment monitoring, environment controlin a clean room, and optical communications.

1. An optical measurement apparatus comprising: a photodetecting portionemitting photoelectrons that correspond to the amount of light in anincident light beam and outputting an output signal that corresponds tothe number of the photoelectrons; a comparing portion comparing theoutput signal with a predetermined threshold value and outputting acomparison result signal when the output signal is greater than thethreshold value, the comparing portion outputting no comparison resultsignal when the output signal is not greater than the threshold value;and a measuring portion performing measurement in accordance with thecomparison result signal, wherein the threshold value is larger than anoutput signal that the photodetecting portion outputs when thephotodetecting portion emits only one photoelectron, the threshold valuebeing smaller than another output signal that the photodetecting portionoutputs when the photodetecting portion emits two photoelectrons,whereby the measuring portion performs measurement when thephotodetecting portion emits two or more photoelectrons, the measuringportion failing to perform measurement when the photodetecting portionemits only one photoelectron.
 2. An optical measurement apparatus asclaimed in claim 1, further comprising a setting portion setting thethreshold value.
 3. An optical measurement apparatus as claimed in claim2, wherein the setting portion measures an output signal that thephotodetecting portion outputs when the photodetecting portion outputs asingle photoelectron, the setting portion setting the threshold valuewithin a range of the amount of the output signal and two times theamount of the output signal.
 4. An optical measurement apparatus asclaimed in claim 2, wherein the setting portion measures, for a fixedperiod of time, output signals that the photodetecting portion outputswhen the photodetecting portion detects no beam of light in a darkstate, the setting portion setting the threshold value to a maximumvalue of the output signals measured during the fixed period of time. 5.An optical measurement apparatus as claimed in claim 1, wherein thethreshold value has a value within a range of 1.2 to 1.8 times an outputsignal that the photodetecting portion outputs when the photodetectingportion emits a single photoelectron.
 6. An optical measurementapparatus as claimed in claim 1, wherein the threshold value has a valuewithin a range of 1.3 to 1.5 times an output signal that thephotodetecting portion outputs when the photodetecting portion emits asingle photoelectron.
 7. An optical measurement apparatus as claimed inclaim 1, wherein the photodetecting portion includes: a photocathodeemitting photoelectrons that correspond to the amount of light in theincident light beam; an accelerating portion accelerating thephotoelectrons emitted from the photocathode; and a semiconductorphotodetector receiving the photoelectrons accelerated by theaccelerating portion and outputting a signal that corresponds to thenumber of the photoelectrons.
 8. An optical measurement apparatus asclaimed in claim 7, wherein the semiconductor photodetector includes anavalanche photodiode.
 9. An optical measurement apparatus as claimed inclaim 1, wherein the comparing portion judges, each time the comparingportion receives a single output signal, whether or not the value of theoutput signal exceeds the threshold value, the comparing portionoutputting a single pulse signal when the comparing portion determinesthat the output signal exceeds the threshold value, thereby indicatingdetection of a single event, in which two or more photoelectrons havebeen emitted; wherein the measuring portion includes a counter countingthe number of the pulse signals outputted from the comparing portion.10. An optical measurement apparatus as claimed in claim 1, wherein themeasuring portion includes an analog-to-digital converting portionexecuting an analog-to-digital conversion to convert the output signalsupplied from the photodetecting portion from analog to digital, whereinthe comparing portion outputs a trigger signal for triggering theanalog-digital converting portion to convert the output signal fromanalog to digital when the output signal exceeds the threshold value.11. An optical measurement apparatus as claimed in claim 2, wherein thesetting portion includes: a shutter selectively blocking a light beamfrom falling incident on the photodetecting portion, thereby selectivelyputting the photodetecting portion in a dark state; a dark statemeasuring portion measuring output signals that the photodetectingportion outputs in the dark state; a switch disposed between thephotodetecting portion and the comparing portion and the dark statemeasuring portion, the switch selectively supplying the output signalsfrom the photodetecting portion to a selected one of the comparingportion and the dark state measuring portion; and a threshold valuedetermination controlling portion controlling the shutter to place thephotodetecting portion in the dark state, controlling the switch tosupply the output signals from the photodetecting portion to the darkstate measuring portion, thereby causing the dark state measuringportion to measure the output signals from the photodetecting portion inthe dark state, the threshold value determination controlling portiondetermining the threshold value based an the results of the dark-statemeasurement.
 12. An optical measurement apparatus as claimed in claim11, wherein the dark state measuring portion includes a wave heightanalyzer measuring a wave height distribution of the output signalsreceived from the photodetecting portion, the threshold valuedetermination controlling portion determining the threshold value basedon a wave height indicative of a peak in the wave height distribution ofthe output signals.
 13. An optical measurement apparatus as claimed inclaim 11, wherein the dark state measuring portion includes a peakdetector detecting a peak value of the output signals received from thephotodetecting portion, the threshold value determination controllingportion determining the threshold value based on the peak value of theoutput signals.
 14. An optical measurement apparatus as claimed in claim1, further comprising a scintillator converting beta rays emitted froman object of measurement into fluorescent light, wherein thephotodetecting portion receives the fluorescent light, and emitsphotoelectrons that correspond to the amount of the fluorescent light,thereby outputting the output signal, and wherein the comparing portioncompares the output signal with the predetermined threshold value andoutputs a comparison result signal when the output signal is greaterthan the threshold value, the comparing portion outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue, the measuring portion counting the fluorescent light inaccordance with the comparison result signal.
 15. An optical measurementapparatus as claimed in claim 1, further comprising a scattered lightgenerating portion generating scattered light by scattering lightaccording to particles mixed in a sample to be measured, wherein thephotodetecting portion receives the scattered light, and emitsphotoelectrons that correspond to the amount of the scattered light,thereby outputting the output signal, and wherein the comparing portioncompares the output signal with the predetermined threshold value andoutputs a comparison result signal when the output signal is greaterthan the threshold value, the comparing portion outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue, the measuring portion counting the particles in accordance withthe comparison result signal.
 16. A scintillation counter, comprising: ascintillator converting beta rays emitted from an object of measurementinto fluorescent light; a photodetecting portion receiving thefluorescent light, and emitting photoelectrons that correspond to theamount of the fluorescent light, thereby outputting an output signalthat corresponds to the number of the photoelectrons; a comparingportion comparing the output signal with a predetermined threshold valueand outputting a comparison result signal when the output signal isgreater than the threshold value, the comparing portion outputting nocomparison result signal when the output signal is not greater than thethreshold value; and a measuring portion counting the fluorescent lightin accordance with the comparison result signal, wherein the thresholdvalue is larger than an output signal that the photodetecting portionoutputs when the photodetecting portion emits only one photoelectron,the threshold value being smaller than another output signal that thephotodetecting portion outputs when the photodetecting portion emits twophotoelectrons, whereby the measuring portion performs counting when thephotodetecting portion emits two or more photoelectrons, the measuringportion failing to perform counting when the photodetecting portionemits only one photoelectron.
 17. A particle counter, comprising ascattered light generating portion generating scattered light byscattering light according to particles mixed in a sample to bemeasured; a photodetecting portion receiving the scattered light, andemitting photoelectrons that correspond to the amount of the scatteredlight, thereby outputting an output signal that corresponds to thenumber of the photoelectrons; a comparing portion comparing the outputsignal with a predetermined threshold value and outputting a comparisonresult signal when the output signal is greater than the thresholdvalue, the comparing portion outputting no comparison result signal whenthe output signal is not greater than the threshold value; and ameasuring portion counting the particles in accordance with thecomparison result signal, wherein the threshold value is larger than anoutput signal that the photodetecting portion outputs when thephotodetecting portion emits only one photoelectron, the threshold valuebeing smaller than another output signal that the photodetecting portionoutputs when the photodetecting portion emits two photoelectrons,whereby the measuring portion performs counting when the photodetectingportion emits two or more photoelectrons, the measuring portion failingto perform counting when the photodetecting portion emits only onephotoelectron.
 18. An optical measurement method comprising: aphotodetecting step emitting photoelectrons that correspond to theamount of light in an incident light beam and outputting an outputsignal that corresponds to the number of the photoelectrons; a comparingstep comparing the output signal with a predetermined threshold valueand outputting a comparison result signal when the output signal isgreater than the threshold value, the comparing step outputting nocomparison result signal when the output signal is not greater than thethreshold value; and a measuring step performing measurement inaccordance with the comparison result signal, wherein the thresholdvalue is larger than an output signal that the photodetecting stepoutputs when the photodetecting step emits only one photoelectron, thethreshold value being smaller than another output signal that thephotodetecting step outputs when the photodetecting step emits twophotoelectrons, whereby the measuring step performs measurement when thephotodetecting zing step emits two or more photoelectrons, the measuringstep failing to perform measurement when the photodetecting step emitsonly one photoelectron.
 19. An optical measurement method as claimed inclaim 18, farther comprising a setting step setting the threshold value.20. An optical measurement method as claimed in claim 19, wherein thesetting step measures an output signal that the photodetecting stepoutputs when the photodetecting step outputs a single photoelectron, thesetting step setting the threshold value within a range of the amount ofthe output signal and two times the amount of the output signal.
 21. Anoptical measurement method as claimed in claim 19, wherein the settingstep measures, for a fixed period of time, output signals that thephotodetecting step outputs when the photodetecting step detects to beamof light in a dark state, the setting step setting the threshold valueto a maximum value of the output signals measured during the fixedperiod of time.
 22. An optical measurement method as claimed in claim18, wherein the threshold value has a value within a range of 1.2 to 1.8times an output signal that the photodetecting step outputs when thephotodetecting step emits a single photoelectron.
 23. An opticalmeasurement method as claimed in claim 22, wherein the threshold valuehas a value within a range of 1.3 to 1.5 times an output signal that thephotodetecting step outputs when the photodetecting step emits a singlephotoelectron.
 24. An optical measurement method as claimed in claim 18,wherein the comparing step judges, each time the comparing step receivesa single output signal, whether or not the value of the output signalexceeds the threshold value, the comparing step outputting a singlepulse signal when the comparing step determines that the output signalexceeds the threshold value, thereby indicating detection of a singleevent, in which two or more photoelectrons have been emitted; whereinthe measuring step includes a counting step courting the number of thepulse signals outputted from the comparing step.
 25. An opticalmeasurement method as claimed in claim 18, wherein the measuring stepincludes an analog-to-digital converting step executing ananalog-to-digital conversion to convert the output signal supplied fromthe photodetecting step from analog to digital, wherein the comparingstep outputs a trigger signal for triggering the analog-digitalconverting step to convert the output signal from analog to digital whenthe output signal exceeds the threshold value.
 26. An opticalmeasurement method as claimed in claim 18, further comprising afluorescent converting step using a scintillator to convert beta raysemitted from an object of measurement into fluorescent light, whereinthe photodetecting step receives the fluorescent light, and emitsphotoelectrons that correspond to the amount of the fluorescent light,thereby outputting the output signal, and wherein the comparing stepcompares the output signal with the predetermined threshold value andoutputs a comparison result signal when the output signal is greaterthan the threshold value, the comparing step outputting no comparisonresult signal when the output signal is not greater than the thresholdvalue, the measuring step counting the fluorescent light is accordancewith the comparison result signal.
 27. An optical measurement method asclaimed in claim 18, further comprising a scattered light generatingstep generating scattered light by scattering light according toparticles mixed in a sample to be measured, wherein the photodetectingstep receives the scattered light, and emits photoelectrons thatcorrespond to the amount of the scattered light, thereby outputting theoutput signal, and wherein the comparing step compares the output signalwith the predetermined threshold value and outputs a comparison resultsignal when the output signal is greater than the threshold value, thecomparing step outputting no comparison result signal when the outputsignal is not greater than the threshold value, the measuring stepcounting the particles in accordance with the comparison result signal.28. A scintillation counting method, comprising: a fluorescentconverting step using a scintillator to convert beta rays emitted froman object of measurement into fluorescent light; a photodetecting stepreceiving the fluorescent light, and emitting photoelectrons thatcorrespond to the amount of the fluorescent light, thereby outputting anoutput signal that corresponds to the number of the photoelectrons; acomparing step comparing the output signal with a predeterminedthreshold value and outputting a comparison result signal when theoutput signal is greater than the threshold value, the comparing stepoutputting no comparison result signal when the output signal is notgreater than the threshold value; and a measuring step counting thefluorescent light in accordance with the comparison result signal,wherein the threshold value is larger than an output signal that thephotodetecting step outputs when the photodetecting step emits only onephotoelectron, the threshold value being smaller than another outputsignal that the photodetecting step outputs when the photodetecting stepemits two photoelectrons, whereby the measuring step performs countingwhen the photodetecting step emits two or more photoelectrons, themeasuring step failing to perform counting when the photodetecting stepemits only one photoelectron.
 29. A particle counting method,comprising: a scattered light generating step generating scattered lightby scattering light according to particles mixed in a sample to bemeasured; a photodetecting step receiving the scattered light, andemitting photoelectrons that correspond to the amount of the scatteredlight, thereby outputting an output signal that corresponds to thenumber of the photoelectrons, a comparing step comparing the outputsignal with a predetermined threshold value and outputting a comparisonresult signal when the output signal is greater than the thresholdvalue, the comparing step outputting no comparison result signal whenthe output signal is not greater than the threshold value; and ameasuring step counting the particles in accordance with the comparisonresult signal, wherein the threshold value is larger than an outputsignal that the photodetecting step outputs when the photodetecting stepemits only one photoelectron, the threshold value being smaller thananother signal that the photodetecting step outputs when thephotodetecting step emits two photoelectron, whereby the measuring stepperforms counting when the photodetecting step emits two or morephotoelectrons, the measuring step failing to perform counting when thephotodetecting step emits only one photoelectron.